Xe máy rung khi chạy.

Vietnamese does not use articles the way English does. A bare noun like xe máy can mean:

• a motorbike
• the motorbike
• motorbikes in general

The exact meaning depends on context. So Xe máy rung khi chạy is natural without any article.

Classifiers are not always required. A classifier such as chiếc is usually used when you are counting something or pointing it out more specifically, for example:

• một chiếc xe máy = one motorbike
• chiếc xe máy này = this motorbike

In a plain statement like Xe máy rung khi chạy, the bare noun xe máy is perfectly normal.

Here, rung functions as a verb, meaning to vibrate or to shake.

Vietnamese often uses verbs where English might use either a verb or an adjective-like structure. So instead of needing something like is vibrating, Vietnamese can simply say rung.

Because Vietnamese does not need a copula like is in this kind of sentence.

In English, you might say The motorbike is vibrating. In Vietnamese, you can simply say Xe máy rung. The verb rung already carries the idea, so no extra is is needed.

Yes, chạy often means to run, but it has a wider range of meanings than English run.

With vehicles, chạy commonly means:

• to run
• to be running
• to move
• to operate
• to drive/go

So in khi chạy, it refers to the motorbike being in operation or in motion, not to a person literally running on foot.

In this sentence, the natural interpretation is when the motorbike is running / moving.

Vietnamese often leaves the subject of the second part unstated when it is understood from context. So khi chạy is understood to relate back to xe máy.

If you wanted to make the rider explicit, you could say something like:

• Khi tôi chạy xe, xe máy rung = When I ride it, the motorbike vibrates.

Khi means when. It introduces a time clause.

So the structure is:

• Xe máy rung = The motorbike vibrates
• khi chạy = when running / when it is running

Together, khi links the main event and the time/condition in which it happens.

A similar word is lúc, which can also mean when in many contexts, though khi is very common and neutral here.

Yes. That is also correct.

Both of these are natural:

• Xe máy rung khi chạy
• Khi chạy, xe máy rung

The difference is mainly emphasis:

• Xe máy rung khi chạy starts with the main fact.
• Khi chạy, xe máy rung starts with the time condition.

English works similarly with The motorbike vibrates when running vs When running, the motorbike vibrates.

Not necessarily. Đang marks an ongoing action, but it is often omitted when the meaning is already clear.

So:

• Xe máy rung khi chạy = natural and normal
• Xe máy rung khi đang chạy = also possible, with a slightly more explicit while it is running feel

The version without đang is simpler and very common.

It can be either, depending on context.

A bare noun in Vietnamese can be:

• generic: motorbikes vibrate when running
• specific: the motorbike vibrates when running

If you want to make it clearly specific, you can add more detail, for example:

• Xe máy của tôi rung khi chạy = My motorbike vibrates when running
• Chiếc xe máy này rung khi chạy = This motorbike vibrates when running

Rung is a very good and natural word if you mean vibrate.

Depending on the situation, you might also hear:

• rung lắc = shake/vibrate quite noticeably
• lắc = shake/sway
• bị rung = be affected by vibration, though this is less direct than plain rung

So if the bike has a mechanical problem and is vibrating, rung is a solid, natural choice. If the shaking is stronger or rougher, rung lắc may fit better.